Digitizer tablets are well known in the art. In one popular type, the pointing device comprises a coil in a cursor or stylus configuration which is positioned by a user over a tablet surface in which is embedded a wire grid extending in two coordinate directions. In one mode, the pointing device coil is energized to electromagnetically induce signals in the grid wires. In another mode, the grid wires are energized to electromagnetically induce signals in the pointing device coil. In both modes, the grid wires are sequentially addressed, either one at a time or in groups, to provide an output analog voltage in timed relation to the grid addressing. The output voltage with increasing time reaches a maximum, passes through zero, and then reaches a minimum in the vicinity of the pointing device coil. When addressing begins, a high frequency counter is activated to interpolate between the particular grid lines addressed. A stop signal for the counter is generated when the zero crossover is determined. The count value determines the location of the pointing device between the two closest grid wires. Examples of patents describing in more detail this type of digitizer are Kamm et al No. 3,904,822; Ioanau No. 3,873,770; and Zimmer No. 4,368,351.
In many of the known systems, analog processing of the output signals is performed. Typically, the output signal is phase displaced with respect to a reference signal which energizes the grid or coil, and the phase displacement is detected and converted to a polarity reversal as the scanned grid wires pass under the pointing device coil in order to more accurately locate the zero crossing point. The resolution of this system is limited by the speed of the counter and the grid spacing.
The trend in the art is to reduce the cost and complexity of digitizer tablets while retaining the same accuracy and resolution, or to increase accuracy and resolution without increasing cost. This has led to an increasing use of digital signal processing circuits. Moreover, with digital signal processing, it becomes easier to add new features to improve user performance or to expand tablet applications.